The present invention relates to a mobile communication system for performing wireless communication by connecting a base station and a mobile station to each other using a TDD (Time Division Duplex) system such as a digital personal handyphone system and a simplified type personal handyphone system. More particularly, the present invention relates to a system for effectively utilizing wireless frequency by improving an inter-base-station frame synchronous system.
Recently, a simplified type personal handyphone system (hereinafter, briefly called a PHS) using a micro cell has been developed as one type of digital mobile communication systems. This simplified type personal handyphone system will be practically used in the near future.
FIG. 1 is a view showing the basic construction of a conventional PHS. Plural PHS base stations CS1 to CSm are dispersively arranged in a service area. Wireless zones Z1 to Zm called cells each having a radius from 100 to 500 meters are formed by these PHS base stations CS1 to CSm. Each of the PHS base stations CS1 to CSm is connected to an integrated service digital network (hereinafter, briefly called an ISDN) INW having a PHS connecting device PM.
Each of mobile stations PS1 to PSn is selectively connected to each of the PHS base stations CS1 to CSm through a wireless channel within the wireless zones Z1 to Zm formed by the above PHS base stations CS1 to CSm. Each of the mobile stations PS1 to PSn is connected to each of wire telephones TEL1 to TELk through the above ISDN or this ISDN and a subscription phone network SNW from these PHS base stations CS1 to CSm. Direct communication can be also performed between the mobile stations PS1 to PS2 by performing connecting control in the PHS base stations CS1.
The PHS has a control center CC having a database, a customer information management database, etc. Information relative to the above mobile stations PS1 to PSn and the PHS base stations CS1 to CSm are collected into this control center CC through the ISDN and a packet network PNW. Service management and control such as authentication, accounting, network management, etc. are performed on the basis of this information.
The PHS adopts a time division multiple access (TDMA) system as an access system between the PHS base stations CS1 to CSm and the mobile stations PS1 to PSn. Further, the PHS adopts a time division bidirectional multiplex (TDD: Time Division Duplex) system as a transmission system.
For example, as shown in FIG. 2, a frame of the TDMA-TDD system is constructed by a forward link and a reverse link. In the forward link, four transmitting slots T1 to T4 are time-divided and multiplexed. In the reverse link, four receiving slots R1 to R4 are time-divided and multiplexed. One frame length is set to 5 ms and a transmission speed in transmission and reception is set to 384 kbps. One slot length is set to about 625 xcexcsec and a transmission speed of information per one slot is set to 32 kbps except for an error correction code, etc.
In calling-out of each of the mobile stations PS1 to PSn, each of the PHS base stations CS1 to CSm transmits timing information of the above TDMA-TDD frame to each of the mobile stations existing within the wireless zones Z1 to Zm of the PHS base stations CS1 to CSm, and synchronizes transmission/reception timing of each of the mobile stations with transmission/reception timing of its own station. In this state, an idle slot within the frame and an idle wireless frequency are allocated to each of the mobile stations as a channel for communication. Thereafter, wireless communication is performed between each PHS base station and each mobile station by using this channel for communication. Accordingly, with respect to each of the PHS base stations CS1 to CSm, the wireless communication can be performed such that the plural mobile stations located within the wireless zones Z1 to Zm of the PHS base stations CS1 to CSm are operated in synchronization with each other and do not come into collision with each other.
However, in the PHS, each of the PHS base stations CS1 to CSm generally generates the TDMA frame independently and performs the communication. Accordingly, no synchronization of the TDMA frame is performed between the PHS base stations CS1 to CSm. Therefore, there is a case in which an interference is caused between the plural base stations of which the wireless zones are adjacent to each other. For example, a transmitting signal of the transmitting slot T1 of an adjacent base station CSa is received in the receiving slot R4 of a base station CSb between the base stations CSa and CSb performing communication in timing shown in FIG. 3. Therefore, an interference is caused in the slot R4 of the base station CSb. To avoid this interference, another slot must be used instead of the slot causing the interference, or a wireless frequency used in the slot causing the above interference must be changed to another frequency.
However, the number of slots of one frame is set to only four in each of transmission and reception. Therefore, interference cannot be avoided by changing the above slots in many cases so that the frequency change must be used. Namely, since no frames are synchronized with each other between the PHS base stations, the channels for communication which were originally usable, become unusable. As a result, utilization efficiency of the communication channel is reduced. This reduction is not very preferable since this reduction causes an increase in lost-call rate when the number of subscribers is increased.
Therefore, there are conventionally several proposals for synchronizing frames with each other between the PHS base stations. Frame synchronous systems provided by these proposals are first classified into a fixed connecting system and an autonomous connecting system in view of synchronous connection.
(A) Fixed Connecting System
This system is a system for predetermining from which base station a certain base station obtains a timing signal for synchronization. Since a position of each base station is known, a propagation delay time between the base stations can be approximately calculated so that the delay time can be simply corrected. However, different data must be inputted and set to each base station. Further, when a new base station is arranged and an arranging position of the base station is changed, a drawback exists in that it is necessary to re-input and reset data for every change.
(B) Autonomous Connecting System
This system is a system for selecting a base station obtaining a timing signal for synchronization on the basis of a certain constant rule. For example, four base stations are selected in the order of larger receiving levels and its average timing is selected. In this system, it is not necessary to input different data for every base station and it is possible to easily cope with situations in which a new base station is arranged and its arranging position is changed. However, it is necessary to repeatedly make calculations for determining timing until the synchronization of an entire system is converged. Further, it is difficult to correctly know a delay time.
Next, the frame synchronous systems are divided into a mutual synchronous system and a master-slave synchronous or master-slave and mutual hybrid synchronous system in view of a synchronous hierarchy.
(1) Mutual Synchronous System
This system is a system for determining frame timing of its own station based on a receiving signal from a mutually adjacent station without arranging a master station as a synchronous source. In this system, problems exist in that it takes much time to converge synchronization since the base stations are mutually influenced, and a converging state is influenced by a delay time.
(2) Master-slave Synchronous or Master-slave and Mutual Hybrid Synchronous System
This system is a system in which a master station as a synchronous source is arranged and all other base stations (slave stations) are operated in synchronization with the master station. Perfect master-slave synchronization is set if all the base stations are operated by a synchronous hierarchy in synchronization with each other based on the timing of an upper hierarchy base station. Further, hybrid synchronization is set if all the base stations are operated in synchronization with stations of the upper hierarchy and/or the same order hierarchy. For example, the master station has a time measuring system using a satellite position measuring system (GPS) and determines a frame delimiter with a time signal obtained by this time measuring system as a trigger signal. In this system, synchronization is stable, but it is necessary to take measures, etc. for a case in which the master station is broken.
Specific examples of synchronous systems by combining the above two classified systems are proposed in papers. Brief explanations and problems of these different synchronous systems will next be described.
(a) Autonomous Connecting Mutual Synchronous System
In this system, burst signals transmitted from each of plural surrounding base stations are received and a weighted average value of the rising timing of a frame/slot is calculated on the basis of a receiving electric field intensity of this burst signal. Timing of the transmitted burst signal of its own station is determined on the basis of this calculated value and is transmitted. Each of the surrounding base stations also receives plural burst signals and similarly calculates a weighted average value with the receiving electric field intensity, and determines timing of the transmitted burst signal of its own station from this calculated weighted average value. The above process is repeated until an error in transmission timing of its own station converges within a certain constant range in each base station.
This system is described in detail in xe2x80x9cY. Akaiwa et al: xe2x80x9cAutonomous Decentralized Inter-base-station for TDMA Microcellular Systemsxe2x80x9d Proceeding IEEE VTC+ 91, PP. 257-262, May 1991xe2x80x9d.
In this system, each base station is autonomously operated so that merits exist in that no control using a control center is required, etc. However, this system has the following various kinds of problems. (1) A dedicated receiver for monitoring the plurality of surrounding base stations at any time is required so that the base stations are large-sized and cost thereof is increased. (2) Time required in which the error is converged in frame timing between the base stations to a constant value is increased as the number of base stations is increased. (3) There is a difficult problem with respect to stability until frame timings of all the base stations are converged by a repeating operation of the above process. (4) Since a correcting amount for correcting dispersion of the propagation delay time caused by the difference in distance between the base stations is a constant value, no frame timing precision can be secured after the convergence.
(b) Fixed Connecting Mutual Synchronous System
This system is a system proposed such that defects of the above autonomous connecting mutual synchronous system are compensated. This system is basically the same system as this autonomous connecting mutual synchronous system. However, in this system, each base station does not monitor all transmitting burst signals of other base stations around its own station, but monitors only plural base stations determined in advance and performs synchronization. Therefore, place coordinates of the base stations as monitored objects around its own station and place coordinates of its own station are inputted to each base station in advance. Each base station checks an ID number of the base station included in a received burst signal, calculates the distance between both the base stations from the place coordinates of its base station and the place coordinates of its own base station, and determines transmitting frame timing of its own station by correcting the propagation delay time on the basis of this calculated distance value.
This system is described in detail in xe2x80x9cH. Kazama et al: xe2x80x9cSemi-Autonomous Synchronization among Base station for TDMA-TDD Communication systemsxe2x80x9d, IEICE Trans. Commun., Vol. E77-B, pp. 862-867, July, 1994xe2x80x9d.
In this system, correcting precision of the propagation delay time is improved so that merits exist in that frame timing is extremely accurate after convergence. However, this system has the following various kinds of problems. Namely, (1) a dedicated receiver for monitoring a partner base station at any time is required so that the cost of a product is increased. (2) There is a difficult problem with respect to converging stability since a connection path will be missing when this synchronous system is disturbed as in regional power supply interruption, etc. (3) It is necessary to provide a large amount of individual information such as the correct place coordinates of surrounding base stations, etc. to each base station in advance so that it is troublesome to manage this information.
(c) Autonomous Connecting Master-slave Synchronous System
In this system, a master base station as a source in synchronous setting and slave base stations for receiving a transmitting burst signal of this master base station and performing synchronization based on this burst signal are determined in advance. Further, slave base station groups are hierarchically formed. At the beginning of synchronization, the master station is set to an uppermost hierarchy order and the slave stations are set to lowermost hierarchy order. Each base station is set to be operated in synchronization with signals of upper stations higher than its own station among receiving signals. The systematic synchronization is autonomously advanced by reducing the hierarchy of its own station from its upper station by one stage and resetting this hierarchy. For example, the master base station as an uppermost station determines frame timing on the basis of high precision time information from a GPS.
This system is described in detail in xe2x80x9cJ. Chang: xe2x80x9cAutonomous Time Synchronization Among Radio Ports in Wireless Personal Communicationsxe2x80x9d, IEEE Trans. VT, Vol. 43, No. 1, pp. 27-32, Feburary 1994xe2x80x9d.
In this system, it is not necessary to perform mutual synchronization so that merits exist in that a synchronous converging time is shortened, etc. from (a) and (b). However, this system has the following various kinds of problems. Namely, (1) a dedicated receiver for monitoring surrounding base stations at any time is required and the hierarchy of its own station must be transmitted together with a synchronous signal at any time so that a product is large-sized and cost thereof is increased. (2) Measures for coping with problems in practical use (for example, a method for coping with a case in which no master base station can transmit a signal by an unexpected accident, etc.) are not clear. (3) When the number of hierarchies is increased, there is a limit in hierarchy since errors in propagating delay time increase with each other in accordance with the distance between the base stations. (4) Since repeat calculation must be carried out, synchronous convergence time is relatively longer.
The autonomous connecting master-slave synchronous system among the above inter-base-station frame synchronous systems is most preferable since stable synchronization can be expected and is not influenced by arranging a new base station and discontinuing a base station. However, in this autonomous connecting master-slave synchronous system, it is necessary to carry out operations of controlling and transmitting the synchronous hierarchy until convergence so that the construction of the base station becomes complicated.
The inventors in this application proposed an inter-base-station frame synchronous system capable of solving these disadvantages in Jpn. Pat. Appln. KOKAI Publication No. 8-289359.
However, in techniques of this Jpn. Pat. Appln. KOKAI Publication No. 8-289359, each base station generates frame timing of its own station in an arbitrary frame synchronous control period among plural frame synchronous control periods set in time division. Therefore, when frame timing on an upper base station is completely generated, no generation processing of the frame timing can be started until the next frame synchronous control period is started. Accordingly, much time is required until synchronizations of all the base stations are performed.
As explained above, the conventionally proposed inter-base-station frame synchronous systems have various kinds of disadvantages.
A first object of the present invention is to provide an inter-base-station frame synchronous system of a mobile communication system capable of simply performing frame synchronization between base stations by autonomous master-slave synchronization without using the control of a synchronous hierarchy, and also provide a base station apparatus applying this system thereto.
A second object of the present invention is to provide an inter-base-station frame synchronous system of a mobile communication system capable of performing frame synchronization without interrupting communication when a base station is on communication.
A third object of the present invention is to provide an inter-base-station frame synchronous system of a mobile communication system, which is little influenced by a propagation delay between base stations and a circuit delay within the base stations.
To achieve the first object, an inter-base-station frame synchronous system of the present invention comprises: plural base stations dispersively arranged within a service area and connected to a communication network, and having at least one master base station and plural slave base stations; and plural mobile stations wirelessly connected to the plural base stations by a time-division bidirectional multiplex system, wherein the master base station comprises: time information receiving means for receiving information with high precision externally supplied; reference frame timing generating means for generating reference frame timing for the time-division multiple connection in synchronization with predetermined synchronous setting timing set on the basis of the time information; and first control channel signal transmitting means for transmitting a control channel signal to the plural slave base stations located around its own station in synchronization with the reference frame timing, wherein the slave base stations comprise: control channel signal observation period setting means in which a predetermined period from a first receiving time point of a control channel signal transmitted from the master base station or other slave base stations is set to a control channel signal observation period; its own station frame timing generating means for generating its own station frame timing on the basis of receiving timing of the control channel signal transmitted from the master base station or one of the other slave base stations when only this control channel signal is received during the control channel signal observation period set by the control channel signal observation period setting means, the its own station frame timing generating means also generating its own station frame timing on the basis of one of receiving timings of control channel signals respectively transmitted from the master base station or plural slave base stations among the other slave base stations when these control channel signals are received during the control channel signal observation period; and second control channel signal transmitting means for transmitting each control channel signal in synchronization with the its own station frame timing after the control channel signal observation period is terminated. A base station apparatus dispersively arranged within a service area and used as a slave base station among plural base stations wirelessly connected to plural mobile stations by a time-division bidirectional multiplex system, the base station apparatus according to the present invention comprises: control channel signal observation period setting means in which a predetermined period from a-first receiving time point of a control channel signal transmitted from other base stations is set to a control channel signal observation period; its own station frame timing generating means for generating its own station frame timing on the basis of receiving timing of the control channel signal transmitted from one of the other base stations when only this control channel signal is received during the control channel signal observation period, the its own station frame timing generating means also generating its own station frame timing on the basis of one of receiving timings of control channel signals respectively transmitted from plural base stations among the other base stations when these control channel signals are received during the control channel signal observation period; and control channel signal transmitting means for transmitting each control channel signal in synchronization with the its own station frame timing after the control channel signal observation period is terminated.
In each of the above slave base stations, the its own station frame timing generating means generates its own station frame timing in synchronization with the frame timing of a most advanced phase among plural frame timings based on the respective receiving timings of the control channel signals respectively transmitted from the master base station or plural slave base stations among the other slave base stations when these control channel signals are received during the control channel signal observation period. In this system, when the control channel signals respectively transmitted from the master base station or plural slave base stations among the other slave base stations are received, the frame timing of a most advanced phase among the plural frame timings based on the respective receiving timings of these control channel signals is discriminated as first frame timing within a period set at a frame interval from a time point shifted by a time about half this frame interval from a first receiving time point of the control channel signals during the control channel signal observation period.
In each of the above slave base stations, as a different system for generating frame timing of its own station in the reception of control channel signals from plural base stations, the its own station frame timing generating means generates its own station frame timing based on receiving timing at a maximum receiving level among the control channel signals respectively transmitted from the master base station or plural slave base stations among the other slave base stations when these control channel signals are received during the control channel signal observation period. Further, this different system can be also constructed by a system, in which an average value of receiving levels of the received plural control channel signals is calculated every received plural control channel signals, and a control channel signal having the average value of receiving levels equal to or greater than a predetermined level is selected and the frame timing of its own station is generated on the basis of an average value of receiving timings.
To achieve the above first object, the base station apparatus of the present invention performs only signal reception for all times in a frame synchronous control period begun from the approximately same time point as predetermined synchronous setting timing of the master station to completion of the generation of the frame timing of at least its own station. A predetermined period from a first receiving time point of a control channel signal transmitted from other base stations during the above frame synchronous control period is set to a control channel signal observation period. The frame timing of its own station is generated on the basis of receiving timing of the control channel signal transmitted from one of the above other base stations when only this control channel signal is received during this control channel signal observation period. The frame timing of its own station is also generated on the basis of one of receiving timings of control channel signals respectively transmitted from plural base stations among the above other base stations when these control channel signals are received during the above control channel signal observation period. Each control channel signal is transmitted to the other base stations around its own station in synchronization with this generated its own station frame timing.
In accordance with these means, reference frame timing is generated in accordance with high precision time information in the master base station, and the control channel signal is transmitted to surrounding slave base stations in synchronization with this reference frame timing. In contrast to this, each slave base station performs only signal reception for all times in the frame synchronous control period from the approximately same time point as predetermined synchronous setting timing to the completion of generation of the frame timing of at least its own station. The frame timing of its own station is generated on the basis of the receiving timing of a control channel signal transmitted from other base stations during the control channel signal observation period as a predetermined period from a time point at which the control channel signal transmitted from the above other base stations is first received during this frame synchronous control period. The control channel is further transmitted to surrounding slave base stations in synchronization with this generated frame timing of its own station. Namely, a field of the frame synchronization is stepwise widened to the surrounding slave base stations in accordance with distances from the master base station.
Therefore, a frame synchronous operation using an autonomous connecting master-slave synchronous system without integrative calculation is performed so that time required to perform the synchronization is shortened.
Further, for example, the generating operation of the reference frame timing in the master base station and the frame synchronous control operation in each slave base station are intermittently performed every 24 hours. Therefore, it is not necessary to receive the control channel signal for the frame synchronization transmitted from a base station on an upper hierarchical side at any time. Accordingly, no dedicated receiver for the frame synchronization is required so that the construction of a base station is simply made compact and cost of a product can be reduced. Further, it is not necessary to control the hierarchy of its own station at any time and transmit this hierarchy together with a control signal at any time so that control can be correspondingly simplified.
To achieve the second object, the inter-base-station frame synchronous system of the present invention is provided that, when the communication between base and mobile stations is approximately performed at the same time point as ordinary frame synchronous control period, the frame synchronous system further comprises frame synchronous starting extending means for setting a frame synchronous control period after the communication between base and mobile stations is terminated, in the slave base stations.
Accordingly, when starting of the frame synchronous control period is delayed, it is desirable that the second control channel signal transmitting means adds information showing whether the communication between base and mobile stations is performed or not or information showing whether it is suitable as reference of the frame synchronous control or not to the control channel signal, and the control channel signal with the information is transmitted to the other slave base stations.
When the starting of the frame synchronous control period is delayed, its own station frame timing generating means desirably generates its own station frame timing based on the receiving timing of only a control channel signal to which information showing that the communication between base and mobile stations is performed or information prohibiting becoming to reference of the frame synchronous control, is not added. Further, the frame synchronous system further comprises talking connection request reception inhibiting means for inhibiting the reception of a talking connection request in at least a frame synchronous control period.
In accordance with such means, when the ordinary frame synchronous control period is started on communication, communication is preferentially performed and the frame synchronous control is performed after this communication is terminated. Therefore, no disadvantages of intermediate interruption of the communication are caused. Further, extending information of the starting of the frame synchronous control is contained to a control channel signal from a slave base station in which the base station is on communication and so the starting of the frame synchronous control is extended. This information is then transmitted from this slave base station. Therefore, in surrounding slave base stations, the above control channel signal, i.e., a signal showing that no frame synchronization is terminated, can be removed from a signal for the frame synchronization. Thus, its own station frame timing can be generated in correct timing at any time.
To achieve the third object, the inter-base-station frame synchronous system of the present invention has timing adjusting means in the second control channel signal transmitting means of a slave base station. In this timing adjusting means, transmitting timing of the control channel signal is variably set on the basis of supposed delay information.
The timing adjusting means variably sets the transmitting timing such that a total time of a supposed propagation delay time from a base station transmitting the control channel signal to its own station, a detecting time required from a receiving time point of this control channel signal to the detection of frame timing shown by this control channel signal, a generating time required from a detecting time point of this frame timing to the generation of its own station frame timing, and a transmitting time until the control channel signal is transmitted in synchronization with this generated its own station frame timing, is an substantially integer times a frame period.
In this case, the propagation delay time is fixedly set to zero or a predetermined value in each slave base station. Otherwise, the propagation delay time is set to be indefinite. The timing adjusting means presumes the propagation delay time of the control channel signal used to generate its own station frame timing on the basis of a receiving level of the received control channel signal. Further, it is possible to apply a system for presuming the propagation delay time of the control channel signal on the basis of a receiving level of the control channel signal used to generate frame timing, and information showing a transmitting level transmitted from a base station on a transmitting side in this control channel, or information of the transmitting level correspondingly stored to this base station in preset transmitting level information every time the control channel signal used to generate the frame timing is received. It is desirable, when the propagation delay time of this control channel signal is presumed on the basis of only the receiving level of the control channel signal, the transmitting level of the control channel signal is set to the same level in each of the master base station and the slave base stations. Further, the timing adjusting means presumes the propagation delay time of the control channel signal used to generate its own station frame timing on the basis of a receiving level of the received control channel signal and information showing a transmitting level transmitted from a base station on a transmitting side in this control channel signal, and transmitting level information storing means for storing transmitting level information showing a transmitting level of the control channel signal in each of the plural base stations, wherein in the received control channel signal used to generate its own station frame timing, the timing adjusting means judges the transmitting level of the received control channel signal with reference to the transmitting level information stored to the transmitting level information storing means on the basis of discriminating information of a base station on a transmitting side transmitted from the base station on the transmitting side in the control channel signal, and presumes the propagation delay time of this control channel signal from the transmitting level and the receiving level.
In accordance with such means, the timing adjusting means is arranged in the second control channel signal transmitting means of each slave base station, and the transmitting timing of the control channel signal is variably set by this timing adjusting means on the basis of the supposed delay information. Therefore, the frame synchronization can be performed with higher precision in consideration of a propagation delay between base stations and a circuit delay generated within the base stations.
As mentioned above in detail, the inter-base-station frame synchronous system of the present invention has at least one master base station and plural slave base stations arranged around this master base station. In the master base station, reference frame timing for a time division bidirectional multiplex is generated in a predetermined synchronous setting time intermittently set on the basis of high precision time information externally received. A predetermined control channel signal is transmitted to the above slave base stations located around its own station in synchronization with this generated reference frame timing. Further, in each slave base station, only signal reception is performed for all times in a frame synchronous control period from the approximately same time point as the above synchronous setting time of the master station to the completion of generation of the frame timing of at least its own station. A predetermined period from a first receiving time point of the control channel signal transmitted from the above master base station or other slave base stations during the above frame synchronous control period is set to a control channel signal observation period. When only the control channel signal transmitted from the above master base station or one of the other slave base stations is received during this control channel signal observation period, the frame timing of its own station is generated on the basis of receiving timing of this control channel signal. When plural control channel signals respectively transmitted from the above master base station or slave base stations among the other slave base stations are received during the control channel signal observation period, the frame timing of its own station is generated on the basis of one of receiving timings of these control channel signals. Each control channel signal is transmitted in synchronization with this generated its own station frame timing.
In the base station apparatus of the present invention, only signal reception is performed for all times in a frame synchronous control period from the approximately same time point as predetermined synchronous setting time of the master station to the completion of generation of the frame timing of at least its own station. A predetermined period from a first receiving time point of a control channel signal transmitted from other base stations during the above frame synchronous control period is set to a control channel signal observation period. When only the control channel signal transmitted from one of the other base stations is received during this control channel signal observation period, the frame timing of its own station is generated on the basis of receiving timing of this control channel signal. When control channel signals respectively transmitted from plural base stations among the other base stations are received during the control channel signal observation period, the frame timing of its own station is generated on the basis of one of receiving timings of these control channel signals. Each control channel signal is transmitted in synchronization with this generated its own station frame timing.
Accordingly, it is possible to provide an inter-base-station frame synchronous system of a mobile communication system capable of simply performing the frame synchronization between base stations by autonomous master-slave synchronization without using the control of a synchronous hierarchy, and also provide a base station apparatus applying this system thereto.
Further, in the inter-base-station frame synchronous system of the present invention, when communication is performed between base and mobile stations approximately at the same time point as the ordinary frame synchronous control period in a slave base station, the frame synchronous control period of the slave base station is established after this communication between base and mobile stations is terminated. Accordingly, it is possible to provide an inter-base-station frame synchronous system of a mobile communication system capable of performing the frame synchronization without interrupting communication when the base station is on communication.
Further, in the inter-base-station frame synchronous system of the present invention, timing adjusting means is arranged in the second control channel signal transmitting means of a slave base station. Transmitting timing of the control channel signal is variably set in this timing adjusting means on the basis of supposed delay information. Accordingly, it is possible to provide an inter-base-station frame synchronous system of a mobile communication system, which is not easily influenced by a propagation delay between base stations and a circuit delay within the base stations.